Technical Field
The present disclosure relates to a method of manufacturing a nitride semiconductor template that is obtained by growing a nitride semiconductor layer made of aluminum nitride or the like on a base substrate being a sapphire substrate.
Description of Related Art
A Group III nitride semiconductor made of gallium nitride or the like is a direct transition type compound semiconductor. Because of its wide band gap and the adjustability of the band gap by the proportion of Group III elements in the composition, the Group III nitride semiconductor is typically used as a solid-state light-emitting element to emit visible light or ultraviolet light.
A Group III nitride semiconductor has a melting point significantly higher than that of silicon or the like, so that it is difficult to obtain a bulk single crystal by using a liquid-phase method such as a melt growth method. Accordingly, generally, a single crystal thin film is obtained on a particular substrate (a base substrate) by using a vapor-phase method such as a chemical vapor deposition (CVD) method. Sapphire is typically used as the base substrate.
However, because of a large difference in lattice constant between sapphire and a Group III nitride semiconductor, a nucleus of a Group III nitride semiconductor is not easily formed on a sapphire substrate. As a result, many defects or dislocations appear in the obtained single crystal thin film of a Group III nitride semiconductor. One of methods for reducing such defects or dislocations is to form a thin film of a specific Group III nitride semiconductor as a buffer layer, before forming a thin film of a desired Group III nitride semiconductor. There is also a case in which a Group III nitride semiconductor layer is further formed on the buffer layer, and the layers from the sapphire substrate to the Group III nitride semiconductor layer may be used collectively as a pseudo-Group III nitride semiconductor substrate. Such a pseudo-Group III nitride semiconductor substrate thus obtained may be referred to as a template or the like. There exist various types of templates and various methods of manufacturing the template.
Meanwhile, in forming various types of Group III nitride semiconductors, a specific impurity (dopant) may be introduced according to the purpose. Oxygen may be selected as such an impurity.
WO 2011/037251 A discloses an embodiment in which, an oxygen source gas such as oxygen gas is supplied with other raw material gas, for the purpose of growing a single crystal of a Group III nitride semiconductor that contains a specific concentration of oxygen, on a sapphire substrate.
JP 2013-004897 A discloses an embodiment in which oxygen is used as a dopant that provides n-type conductivity to a Group III nitride semiconductor layer formed on a gallium nitride substrate, and an embodiment in which water vapor in ammonia gas is used as a raw material of the oxygen.
Meanwhile, in some cases, a specific treatment may be performed on the surface of a base substrate, according to the purpose.
Japanese Journal of Applied Physics 52 (2013) 08JB21 discloses that, after nitriding a surface of a sapphire substrate, growing a layer of aluminum nitride on the nitride surface can achieve an aluminum nitride layer with no cracks.
In recent years, a Group III nitride semiconductor is increasingly drawing attention to be used for a solid-state light-emitting element emitting ultraviolet light. A Group III nitride semiconductor used as an active layer in a solid-state light-emitting element to emit ultraviolet light can be aluminum gallium nitride (AlGaN) with a high proportion of aluminum. Accordingly, in the case where ultraviolet light emitted from the active layer is extracted to the substrate side (i.e., a so-called “face-down” structure), the nitride semiconductor layer on the substrate side of the active layer is made of AlGaN (including AlN for the sake of expediency) which contains a higher ratio of aluminum than the active layer.
In recent years, higher performance is increasingly demanded of a solid-state light-emitting element to emit ultraviolet light, and a further improvement is also demanded of a nitride semiconductor template therefor.